Prerequisites

1. Java Development Kit (JDK) 21 or higher: Ensure JDK is installed and configured on your system.
2. Integrated Development Environment (IDE): IntelliJ IDEA, Eclipse, or any other IDE.
3. Maven: Ensure Maven is installed and configured on your system.

Step 1: Create a Maven Project

1. Open your IDE and create a new Maven project.
2. Update the pom.xml file to include Hibernate and other required dependencies.

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>com.example</groupId> <artifactId>hibernate-many-to-one-example</artifactId> <version>1.0-SNAPSHOT</version> <dependencies> <dependency> <groupId>org.hibernate.orm</groupId> <artifactId>hibernate-core</artifactId> <version>6.2.0.Final</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-api</artifactId> <version>1.7.30</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-simple</artifactId> <version>1.7.30</version> </dependency> <dependency> <groupId>com.h2database</groupId> <artifactId>h2</artifactId> <version>1.4.200</version> </dependency> </dependencies> </project> 

Explanation

• Hibernate Core Dependency: Includes the main Hibernate framework.
• SLF4J Dependencies: Used for logging.
• H2 Database Dependency: An in-memory database for testing purposes.

Step 2: Create Hibernate Configuration File

Create a file named hibernate.cfg.xml in the src/main/resources directory.

<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <property name="hibernate.dialect">org.hibernate.dialect.H2Dialect</property> <property name="hibernate.connection.driver_class">org.h2.Driver</property> <property name="hibernate.connection.url">jdbc:h2:mem:testdb</property> <property name="hibernate.connection.username">sa</property> <property name="hibernate.connection.password"></property> <property name="hibernate.hbm2ddl.auto">update</property> <property name="hibernate.show_sql">true</property> <property name="hibernate.format_sql">true</property> </session-factory> </hibernate-configuration> 

Explanation

• Dialect: Specifies the SQL dialect (H2 in this case).
• Connection Properties: Configure the JDBC connection to the H2 database.
• hbm2ddl.auto: Automatically manages the database schema (update existing schema).
• show_sql: Prints SQL statements to the console.
• format_sql: Formats SQL statements.

Step 3: Create the Department Entity Class

Create a package named com.example.entity and a class named Department.

package com.example.entity; import jakarta.persistence.*; import java.util.HashSet; import java.util.Set; @Entity public class Department { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; @OneToMany(mappedBy = "department", cascade = CascadeType.ALL, orphanRemoval = true) private Set<Employee> employees = new HashSet<>(); public Department() {} public Department(String name) { this.name = name; } public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Set<Employee> getEmployees() { return employees; } public void setEmployees(Set<Employee> employees) { this.employees = employees; } public void addEmployee(Employee employee) { employees.add(employee); employee.setDepartment(this); } public void removeEmployee(Employee employee) { employees.remove(employee); employee.setDepartment(null); } @Override public String toString() { return "Department{id=" + id + ", name='" + name + '\'' + '}'; } } 

Explanation

• @Entity: Marks the class as an entity.
• @Id: Marks the field as the primary key.
• @GeneratedValue: Specifies the strategy for generating values for the primary key.
• @OneToMany: Defines a one-to-many relationship with the Employee entity.
• mappedBy: Specifies the field in the Employee entity that owns the relationship.
• cascade: Specifies the cascade operations.
• orphanRemoval: Specifies whether to remove orphaned entities.

Step 4: Create the Employee Entity Class

Create a class named Employee in the same package.

package com.example.entity; import jakarta.persistence.*; @Entity public class Employee { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; @ManyToOne(fetch = FetchType.LAZY) @JoinColumn(name = "department_id") private Department department; public Employee() {} public Employee(String name) { this.name = name; } public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Department getDepartment() { return department; } public void setDepartment(Department department) { this.department = department; } @Override public String toString() { return "Employee{id=" + id + ", name='" + name + '\'' + '}'; } } 

Explanation

• @Entity: Marks the class as an entity.
• @Id: Marks the field as the primary key.
• @GeneratedValue: Specifies the strategy for generating values for the primary key.
• @ManyToOne: Defines a many-to-one relationship with the Department entity.
• @JoinColumn: Specifies the foreign key column.

Step 5: Create a Hibernate Utility Class

Create a package named com.example.util and a class named HibernateUtil.

package com.example.util; import org.hibernate.SessionFactory; import org.hibernate.boot.registry.StandardServiceRegistryBuilder; import org.hibernate.cfg.Configuration; import org.hibernate.service.ServiceRegistry; public class HibernateUtil { private static SessionFactory sessionFactory; static { try { Configuration configuration = new Configuration().configure(); ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder() .applySettings(configuration.getProperties()).build(); sessionFactory = configuration.buildSessionFactory(serviceRegistry); } catch (Throwable ex) { throw new ExceptionInInitializerError(ex); } } public static SessionFactory getSessionFactory() { return sessionFactory; } } 

Explanation

• Configuration: Loads Hibernate configuration from hibernate.cfg.xml.
• ServiceRegistry: Builds the service registry from the configuration settings.
• SessionFactory: Provides sessions to interact with the database.

Step 6: Create Main Class

Create a package named com.example and a class named Main.

package com.example; import com.example.entity.Department; import com.example.entity.Employee; import com.example.util.HibernateUtil; import org.hibernate.Session; import org.hibernate.Transaction; public class Main { public static void main(String[] args) { // Initialize session and transaction Session session = HibernateUtil.getSessionFactory().openSession(); Transaction transaction = session.beginTransaction(); // Create department Department department = new Department("IT"); // Create employees Employee employee1 = new Employee("John Doe"); Employee employee2 = new Employee("Jane Doe"); // Add employees to department department.addEmployee(employee1); department.addEmployee(employee2); // Save department (this will also save employees due to cascade) session.save(department); transaction.commit(); session.close(); // Retrieve and display department session = HibernateUtil.getSessionFactory().openSession(); Department retrievedDepartment = session.get(Department.class, department.getId()); System.out.println("Retrieved Department: " + retrievedDepartment); System.out.println("Employees: " + retrievedDepartment.getEmployees()); session.close(); // Update an employee session = HibernateUtil.getSessionFactory().openSession(); transaction = session.beginTransaction(); Employee employeeToUpdate = session.get(Employee.class, employee1.getId()); employeeToUpdate.setName("John Smith"); session.update(employeeToUpdate); transaction.commit(); session.close(); // Delete an employee session = HibernateUtil.getSessionFactory().openSession(); transaction = session.beginTransaction(); Department departmentToUpdate = session.get(Department.class, department.getId()); Employee employeeToDelete = session.get(Employee.class, employee2.getId()); departmentToUpdate.removeEmployee(employeeToDelete); session.delete(employeeToDelete); transaction.commit(); session.close(); // Retrieve and display updated department session = HibernateUtil.getSessionFactory().openSession(); Department updatedDepartment = session.get(Department.class, department.getId()); System.out.println("Updated Department: " + updatedDepartment); System.out.println("Employees: " + updatedDepartment.getEmployees()); session.close(); // Close the SessionFactory HibernateUtil.getSessionFactory().close(); } } 

Explanation

• Session: Opens a session to interact with the database.
• Transaction: Begins and commits a transaction for database operations.
• Save: Persists the entity to the database.
• Retrieve: Fetches the entity from the database using its ID.
• Update: Updates the entity in the database.
• Delete: Removes the entity from the database.

Step 7: Run the Application

1. Run the Main class.
2. The output in the console should be:

Hibernate: create table Department (id bigint generated by default as identity, name varchar(255), primary key (id)) Hibernate: create table Employee (id bigint generated by default as identity, department_id bigint, name varchar(255), primary key (id)) Hibernate: alter table Employee add constraint FKk19gk3d8xlt8afm6u7g4s7hn6 foreign key (department_id) references Department Hibernate: insert into Department (name) values (?) Hibernate: insert into Employee (department_id, name) values (?, ?) Hibernate: insert into Employee (department_id, name) values (?, ?) Hibernate: select departmen0_.id as id1_0_0_, departmen0_.name as name2_0_0_ from Department departmen0_ where departmen0_.id=? Hibernate: select employees0_.department_id as departme3_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.department_id as departme3_1_1_, employees0_.name as name2_1_1_ from Employee employees0_ where employees0_.department_id=? Retrieved Department: Department{id=1, name='IT'} Employees: [Employee{id=1, name='John Doe'}, Employee{id=2, name='Jane Doe'}] Hibernate: select employee0_.id as id1_1_0_, employee0_.department_id as departme3_1_0_, employee0_.name as name2_1_0_ from Employee employee0_ where employee0_.id=? Hibernate: update Employee set department_id=?, name=? where id=? Hibernate: select departmen0_.id as id1_0_0_, departmen0_.name as name2_0_0_ from Department departmen0_ where departmen0_.id=? Hibernate: select employees0_.department_id as departme3_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.department_id as departme3_1_1_, employees0_.name as name2_1_1_ from Employee employees0_ where employees0_.department_id=? Hibernate: select employee0_.id as id1_1_0_, employee0_.department_id as departme3_1_0_, employee0_.name as name2_1_0_ from Employee employee0_ where employee0_.id=? Hibernate: delete from Employee where id=? Hibernate: update Employee set department_id=? where id=? Hibernate: select departmen0_.id as id1_0_0_, departmen0_.name as name2_0_0_ from Department departmen0_ where departmen0_.id=? Hibernate: select employees0_.department_id as departme3_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.department_id as departme3_1_1_, employees0_.name as name2_1_1_ from Employee employees0_ where employees0_.department_id=? Updated Department: Department{id=1, name='IT'} Employees: [Employee{id=1, name='John Smith'}] 

Conclusion

You have successfully created an example using Hibernate to demonstrate a many-to-one mapping with annotations. This tutorial covered setting up a Maven project, configuring Hibernate, creating entity classes with a many-to-one relationship, and performing CRUD operations.